feat(design): dialogs open by purpose and keep their commit row pinned (E28 S3f1)

Modal takes a required initialFocus (field / close / action), a field that
loads late takes the focus while it still rests on the close button, and
ModalFooter portals a dialog's commit row under the scrolling body from any
depth. The header and footer fades follow the body's scroll timeline, and the
viewport's interactive-widget=resizes-content keeps the footer above an
on-screen keyboard.

frontend/src/components/ReleaseNotes.tsx

@@ -45,7 +45,12 @@ export default function ReleaseNotes({
   const active = at >= 0 ? highlight : notes[0]?.version;
   const shown = showAll || at >= RECENT ? notes : notes.slice(0, RECENT);
   return (
-    <Modal title={t("about.releaseNotes")} onClose={onClose} maxWidth="max-w-2xl">
+    <Modal
+      initialFocus="close"
+      title={t("about.releaseNotes")}
+      onClose={onClose}
+      maxWidth="max-w-2xl"
+    >
       {q.isError ? (
         <StateMessage tone="error" onRetry={() => q.refetch()}>
           {t("about.notesError")}